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Lorentz about QM

This is an excerpt of the opening statement of Hendrik Antoon Lorentz from the "general discussion" session of the 1927 Solvay conference. (Excerpt taken from Bacciagaluppi, Valentini - Quantum Theory at the Crossroads - Reconsidering the 1927 Solvay Conference.)

I should like to draw attention to the difficulties one encounters in the old theories.

We wish to make a representation of the phenomena, to form an image of them in our minds. Until now, we have always wanted to form these images by means of the ordinary notions of time and space. These notions are perhaps innate; in any case, they have developed from our personal experience, by our daily observations. For me, these notions are clear and I confess that I should be unable to imagine physics without these notions. The image that I wish to form of phenomena must be absolutely sharp and definite, and it seems to me that we can form such an image only in the framework of space and time.

For me, an electron is a corpuscle that, at a given instant, is present at a definite point in space, and if I had the idea that at a following moment the corpuscle is present somewhere else, I must think of its trajectory, which is a line in space. And if the electron encounters an atom and penetrates it, and after several incidents leaves the atom, I make up a theory in which the electron preserves its individuality; that is to say, I imagine a line following which the electron passes through the atom. Obviously, such a theory may be very difficult to develop, but a priori it does not seem to me impossible.

I imagine that, in the new theory, one still has electrons. It is of course possible that in the new theory, once it is well-developed, one will have to suppose that the electrons undergo transformations. I happily concede that the electron may dissolve into a cloud. But then I would try to discover on which occasion this transformation occurs. If one wished to forbid me such an enquiry by invoking a principle, that would trouble me very much. It seems to me that one may always hope one will do later that which we cannot yet do at the moment. Even if one abandons the old ideas, one may always preserve the old classifications [dénominations]. I should like to preserve this ideal of the past, to describe everything that happens in the world with distinct images. I am ready to accept other theories, on condition that one is able to re-express them in terms of clear and distinct images.

In his "Lectures on Theoretical Physics" Lorentz describes his ideas about light quanta.

The hypothesis of light-quanta [...] is in contradiction with the phenomena of interference. Can the two views be reconciled? I should like to put forward some considerations about this question, but I must first say that Einstein is to be given credit for whatever in them may be sound. As I know his ideas concerning the points to be discussed only by verbal communication, however, and even by hearsay, I have to take the responsibility for all that remains unsatisfactory.

Let us suppose that in the emission and propagation of light there is something that conforms wholly to Maxwell's equations, but that it has practically no energy at all, the electric and magnetic [fields] being infinitely small. Then in this [...] radiation we shall have the ordinary laws of reflection, interference, and refraction, but we shall see nothing of it. On a screen you will have something like an undeveloped photographic image.

We can now imagine that in the production of light this [...] radiation is accompanied by the emission of certain quanta of energy that are of a different nature. Although their precise nature is unknown, we may suppose that energy is concentrated in small spaces and remains so. These quanta move in such a way in our "pattern" that they can never come to a place where in this pattern there is darkness. In thus traveling from the source outward each quantum has a choice between many paths. The probability of following different paths is proportional to the intensity of the radiation along these paths [...].

Now in all real cases the act of emission is repeated a great many times. Suppose it is repeated N times and let the [...] radiation be the same in these different cases. Then we shall have N quanta moving in this pattern, and if their number is very great and the probability of following different paths as stated, the number of quanta coming on different parts of a screen on which we observe an interference phenomenon will be proportional to the intensity which we have in [the wave] pattern.

These considerations can easily be extended. Take, for instance, polarization. The polarization will be in the [wave] pattern, not in the quanta, but the quanta will illuminate a screen or a photographic plate or our retina to exactly the degree determined by the classical theory.

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